1. Field of the Invention
The present invention relates to an electric system for an electric vehicle comprising a secondary battery as a power supply.
2. Description of the Related Art
FIG. 1 shows a power train for a conventional electric vehicle having a secondary battery as its power supply. Each of the wheels of the electric vehicle is driven separately by an AC motor.
In this figure, reference numeral 1 designates a secondary battery; 2, a main switch; 31 and 32, fuses; 41 and 42, three-phase inverters for use as a power converter having a regenerative function; 51 and 52, AC motors; 61 and 62, connecting wires connecting the inverters 41 and 42 and the AC motors 51 and 52, respectively; 71 and 72, reduction gears; and 81 and 82, wheels. The rotations of the AC motors 51 and 52 are reduced by the reduction gears 71 and 72, and are transmitted to the wheels 81 and 82.
FIG. 2 shows a power train which drives two wheels at the same time by one AC motor. In this figure, reference numeral 3 denotes a fuse; 4, an inverter; 5, an AC motor; 6, connecting wires; 7, a reduction gear; and 9, a differential gear. The other components are the same as those in FIG. 1.
In the drive systems in FIGS. 1 and 2, each of the inverters 41, 42 and 4 converts the DC power of the secondary battery 1 to the AC power so as to control the torque and the rotation rates of the AC motors 51, 52 and 5.
In the motoring mode of the electric vehicle, the power of the secondary battery 1 is DC-to-AC converted by the inverters, and is supplied to the motors from the inverters, thus driving the wheels. In contrast with this, in the regenerative braking mode, braking is performed by rectifying AC to DC by the inverters so that the kinetic energy of the electric vehicle is regenerated as the DC power to the secondary battery 1 via the wheels, motors and inverters.
FIG. 3 shows a three-phase transistor inverter employed as the inverters of the electric vehicle. Although the inverter 4 used in the drive system of FIG. 2 is explained in the description below, the inverters 41 and 42 in the drive system of FIG. 1 are similar.
In FIG. 3, reference numeral 401 denotes transistors, and 402 designates diodes, each of which is connected in antiparallel with each one of the transistors 401. The main circuit of the three-phase inverter comprises six arms each of which includes the transistor 401 and the diode 402. Reference numeral 403 denotes a capacitor for smoothing the current of the secondary battery 1.
FIGS. 5A-5D and 6A-6D illustrate the waveforms of the input and output currents and voltages of the inverter while driving the electric vehicle. Generally, inverters for driving an electric vehicle employ PWM (pulse width modulator) control like inverters employed for driving AC motors widely used in industry. The inverter 4 of FIG. 3 also uses PWM control. The voltages V.sub.M, V.sub.B and the currents i.sub.M, i.sub.B are identified in FIG. 4, FIGS. 5A-5D illustrate the voltage and current waveforms in the motoring mode, and FIGS. 6A-6D illustrate these waveforms in the regenerative braking mode. As seen from FIGS. 5A-5D and 6A-6D, the AC side voltage V.sub.M of the inverter 4 has a waveform obtained by performing the PWM control on the voltage V.sub.B of the secondary battery 1. This waveform is similar in both the motoring mode and the regenerative braking mode. The dotted curve shown in the waveform of the voltage V.sub.M indicates the fundamental wave of the PWM control. The PWM control performs such control as the fundamental wave becomes a sine-wave. The AC side current i.sub.M of the inverter 4 has a waveform in which a higher harmonic current is superimposed on the sinusoidal fundamental wave.
The waveforms shown in FIGS. 5A-5D and 6A-6D indicate that the power factor is 1.0 in this case. As shown in these figures, the phase of the current i.sub.M in the regenerative braking mode is opposite to that in the motoring mode so that the regenerative operation is performed. The current i.sub.B at the DC side of the inverter 4 is also reversed in the regenerative braking mode.
Since the stored energy in the secondary battery of the electric vehicle is limited, it must be charged at times, and this is essential in using the electric vehicle. In other words, the charging of the secondary battery and a charging system are essential in using the electric vehicle.
FIG. 7 shows a conventional charging system. In this figure, reference numeral 100 designates an electric vehicle comprising the same elements as shown in FIG. 2.
In FIG. 7, reference numeral 300 denotes a charging system which is connected to a charging connector 200 via charging cables 400 at the DC side, and to a connector 600 via cables 700 at the AC side. The charging connector 200 is connected to the secondary battery 1, and the connector 600 is connected to a distribution network 500.
When charging the secondary battery 1, the main switch 2 in the electric vehicle 100 is opened, and the secondary battery 1 is charged with the DC power supplied from the charging system 300 which rectifies the AC power supplied from the distribution network 500.
FIG. 8 illustrates a conventional charging system 300. In FIG. 8, reference numeral 301 designates a switch at the AC side of the system; 302, a step-down transformer provided as needed; 303, a rectifier made of diodes which rectifies an AC voltage into a DC voltage; 304, a chopper for controlling the charging current; 305, a reactor for smoothing the charging current; and 306, a fuse.
The charging system 300 is usually required to quickly charge the secondary battery 1 except when it is allowed to take enough time for charging. Accordingly, the capacity of the charging system must be at least equal to that of the inverter for driving the AC motor of the electric vehicle. Thus, since the charging system must have a large capacity and incorporate a power converter like the diode rectifier 303, the dimension of the charging system becomes large, and the charging operation requires a large amount of space.
FIG. 9 illustrates the charging operation of the electric vehicle 100.
It is essential to locate charging stations as shown in FIG. 9 at various sites such as gasoline stations for automobiles using internal combustion engines so that the electric vehicle can run without restriction of time and space.
In the conventional charging system, however, since the charging system 300 is bulky and expensive, it is difficult to locate it at many spots, and this presents a problem in increasing the use of electromobiles.
In addition, since the charging system 300 has a rectifying load which is to be connected to the distribution network 500, it induces higher harmonics on the distribution network 500, or reduces the power factor. This presents a problem in that the quality of the power supplied by the distribution network is deteriorated.